The present invention relates to exhaust systems for internal combustion engines, and more particularly to a system for decomposing reductant and mixing together reductant and exhaust gas in a selective catalytic reduction (SCR) catalyst device of an exhaust aftertreatment system.
Exhaust aftertreatment systems receive and treat exhaust gas generated from an internal combustion engine such as a diesel engine. Typical exhaust aftertreatment systems include any of various devices configured to reduce the level of unwanted exhaust emissions present in the exhaust gas. Some exhaust aftertreatment systems for diesel powered internal combustion engines include various devices, such as a diesel oxidation catalyst (DOC), particulate matter filter or diesel particulate filter (DPF), and a selective catalytic reduction (SCR) catalyst device. In some exhaust aftertreatment systems, exhaust gas first passes through the diesel oxidation catalyst, then passes through the diesel particulate filter, and subsequently passes through the SCR catalyst.
Each of the DOC, DPF, and SCR catalyst devices is configured to perform a particular exhaust emissions treatment operation on the exhaust gas passing through the devices. Generally, the DOC reduces the amount of carbon monoxide and hydrocarbons present in the exhaust gas via oxidation techniques. The DPF filters diesel particulate matter and soot present in the exhaust gas. Finally, the SCR catalyst device reduces the amount of nitrogen oxides (NOx) present in the exhaust gas.
The SCR catalyst device is configured to reduce NOx into N2 and H2O, in the presence of ammonia (NH3). Because ammonia is not a natural byproduct of the combustion process, it must be artificially introduced into the exhaust gas prior to the exhaust gas entering the SCR catalyst device. Typically, ammonia is not directly injected into the exhaust gas due to safety considerations associated with the storage of liquid ammonia. Accordingly, conventional systems are designed to inject a urea-water solution, or diesel exhaust fluid (DEF) into the exhaust gas, which is capable of decomposing into ammonia in the presence of the exhaust gas. SCR systems typically include a urea source and a urea injector or doser coupled to the source and positioned upstream of the SCR catalyst device.
Generally, the decomposition of the urea-water solution into gaseous ammonia occupies three stages. First, urea evaporates or mixes with exhaust gas. Second, the temperature of the exhaust causes a phase change in the urea and decomposition of the urea into isocyanic acid (HNCO) and water. Third, the isocyanic acid reacts with water in a hydrolysis process under specific pressure and temperature concentrations to decompose into ammonia and carbon dioxide (CO2). The ammonia is then introduced at or near the inlet face of the SCR catalyst device, flows through the catalyst, and is consumed in the NOx reduction process. Any unconsumed ammonia exiting the SCR system can be reduced to N2 and other less noxious components using an ammonia oxidation catalyst.
To sufficiently decompose into ammonia, the injected urea must be given adequate time to complete the three stages. The time given to complete the three stages and decompose urea into ammonia before entering the SCR catalyst device is conventionally termed residence time. Some prior art exhaust aftertreatment systems utilize a long tube of a fixed linear decomposition length that extends between the urea injector and SCR catalyst device inlet face. The fixed linear decomposition length of prior art systems must be quite long in order to provide the necessary residence time. Long tubing for urea decomposition often takes up valuable space that could be occupied by other vehicle components and influences the design of the exhaust aftertreatment system. However, shorter decomposition tubes associated with some prior art end-in, end-out and end-in, side-out SCR systems may not provide a sufficiently long residence time to properly evaporate the injected urea.
Additionally, some prior art exhaust aftertreatment systems, particularly those systems that utilize or require in-line or end-to-end or end-to-side devices, do not provide adequate mixing of the urea/ammonia with the exhaust gas. Inadequate mixing results in a low ammonia vapor uniformity index, which can lead to crystallization/polymerization buildup inside the SCR catalyst device or other SCR system devices, localized aggregation of ammonia, inadequate distribution of the ammonia across the SCR catalyst surface, lower NO conversion efficiency, and other shortcomings.
Further, many exhaust aftertreatment systems with end-to-end or end-to-side SCR systems fail to adequately distribute exhaust gas across the inlet face of the SCR catalyst device. An uneven distribution of exhaust gas at the SCR catalyst device inlet can result in excessive ammonia slip and less than optimal NOx conversion efficiency. For example, a low exhaust flow distribution index at the SCR catalyst device inlet results in a lower amount of SCR catalyst surface area in contact with the exhaust gases. The lesser the catalyst surface area in contact with the exhaust gases, the lower the NOx reduction efficiency of the SCR catalyst device.
What is needed in the art is a more efficient mixing of the exhaust gases prior to or at the inlet of the SCR device.